[css-values-3] Default ch to 1em when text-orientation is upright.

Author: tabatkins

css-values/Overview.bs

@@ -872,6 +872,8 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 
 			In the cases where it is impossible or impractical to determine the measure of the "0" glyph,
 			a value of 0.5em must be assumed.
+			If 'text-orientation' is ''text-orientation/upright'',
+			a value of 1em must instead be assumed.
 
 		<dt><dfn lt="rem">rem unit</dfn>
 		<dd>